Polish discrepances between new and old settings pages

[joedolson]

As discussed in #65100, there are numerous differences in the design of newer settings pages that create tension between different parts of the WordPress admin.

Three prominent differences include

1) The main heading font and style.
2) The background color.
3) The incorporation of a border radius in the content region.

These should be evened out to make the admin more uniform.

@fabian-kaegy has an opened ticket (#64641) already for incorporating some additional changes in core, but the use of the hard white background is a problem for accessibility, and should be softened.

[audrasjb]

Removing trunk version as this is not going to be shipped with WP 7.0 but in the next releases.

[desrosj]

Since this is an enhancement, there's no first version of WordPress this can be reproduced in. Removing trunk version.

[joedolson]

Related: #65590 (for the problems in black/white contrast.)